Properties of olefin polymers are determined by molecular weight distribution and stereoregularity. Among others, olefin polymers having narrow molecular distribution are of high practical utility in the field of injection molding because of their high flexual modules, impact resistance and heat distortion resistance.
Ziegler-Natta catalyst carrying magnesium chloride which is widely employed in industry contains an organosilicon compound as a cocatalyst component (see JP-A-57-63310, JP-A-58-63311, JP-A-58-138708, JP-A-58-83006, JP-A-61-296006, JP-A-62-11705, JP-A-62-18406, JP-A-62-20507, JP-A-63-92615, JP-A-63-175008, JP-A-63-258907, JP-A-63-289004, JP-A-2-70708, JP-A-2-163104, JP-A-2-170803, JP-A-2-173010, JP-A-3-119004, JP-A-3-33102, JP-A-3-33103, JP-A-3-33105, and JP-A-3-33106; the term "JP-A" as used herein means an "unexamined published Japanese patent application").
Olefin polymers produced using these catalyst systems usually have a molecular weight distribution of more than 19 as expressed in terms of a ratio of high-load melt index (HLMFR) measured under a load of 10 kg to melt index (MFR) measured under a load of 2.16 kg, HLMFR/MFR. The higher the HLMFR/MFR, the broader the molecular weight distribution. It is well known that olefin polymers with narrow molecular weight distribution are preferred for production of monofilaments in good demand in industry. For the time being, olefin polymers having narrow molecular weight distribution, i.e., HLMFR/MFR of less than 19, have been produced by visbreaking of an olefin polymer using a peroxide. However, visbreaking is difficult to control so that complicated processing is required and reproducibility is low. Besides, visbreaking of olefin polymers, especially propylene polymers tend to be attended by reduction in stereoregularity.
Some improvements on Ziegler-Natta catalysts have recently been proposed for obtaining olefin polymers with narrower molecular weight distribution as disclosed in JP-A-59-64602, JP-A-59-207904, JP-A-60-1041902, JP-A-61-126110, JP-A-62-104812, JP-A-63-199207, JP-A-1-54008, JP-A-2-70708, and JP-A-3-47806. However, any of the olefin polymers obtained by these processes has a stereoregularity (a proportion of boiling heptane-insoluble matter) of 97% at the highest, still meeting difficulty in reaching high rigidity as demanded today.
On the other hand, it is possible to obtain an olefin polymer with narrow molecular weight distribution by using a Kaminsky type homogeneous catalyst system as described in, for example, JP-A-63-295607. However, since this catalyst system needs a large quantity of very expensive aluminoxane, it has not yet come into industrial use.